In practice, invariably there are heat losses through the filling hole caps, by direct conduction and dissipation, and also otherwise, such as by leakage. Heat losses through filling hole caps are undesirable from more than one viewpoint. Obviously heat losses through the caps represent needlessly wasted energy and consequent loss of money. Furthermore, it has been observed that the caps which are usually made of cast iron or steel, warp because of thermal stress reversal in the course of several loadings and unloadings of a single shaft. The warp results in gaps at the periphery of the cap, further augmenting the heat loss. It is customary in coke oven installations to provide a flexible metallic seal, e.g., of stainless steel sheet metal, to facilitate good sealing action; notwithstanding, the warping suffered by the cast iron or the steel lid causes the metallic seal to be ineffective. As a consequence, not only are the heat losses from the cap increased, but also, hot gases containing a profuse amount of dust and other entrained particles are let out into the atmosphere. In most instances environmental regulations necessitate installation of equipment to filter the hot gases to get rid of the dust and other entrained particles; such installation again results in expenses and additional needless investment without proper returns.
There is yet another consideration why heat conduction and consequent heat loss should be prevented from the filling hole cap. As mentioned earlier, it is known practice to lift the cap magnetically during filling the shaft, as well as during unloading. In the event of totally ferrous lid if a permanent magnet is used for the hoisting or lifting of the cap, the magnet would tend to undergo premature demagnetization because of exposure to high temperatures, as is well known to those who are skilled in the art. If, on the other hand, an electromagnet is used for hoisting, design provisions would have to be made in the electromagnet for exposure to extremely high temperatures; such design provision may be in the form of increased cross sectional area of the conductor of the electromagnetic winding, consequently rendering the equipment relatively expensive. Looked at from various angles, heat conduction, dissipation and consequent heat loss through caps of filling shafts of coke ovens are always undesirable.
Attempts to construct the caps exclusively from ceramic material would result in brittleness and loss of mechanical strength. Besides, a cap made totally of ceramic material would lose the ability of magnetic hoisting, which is a very convenient feature. There has been a need for a cap construction for use with filling holes of coke ovens, which construction permits magnetic hoisting, and at the same time cuts the heat losses to a negligible amount, obviating all the attendant disadvantages. An experimental set-up wherein a metallic cap on a filling hole was completely removed and was replaced by brickwork to seal the cap-opening indicated that the heat losses through the cap area were minimal when there was no metal around the cap or in the cap whatsoever, which was directly exposed to the inside of the coke oven. It seemed extremely desirable to extend the heat loss prevention to the area around the cap also; accordingly, a frame built of ceramic material or other firebricks was tried out in conjunction with a cap constructed according to this invention, resulting in further reduction of heat loss. Repeated opening and closing of the cap in an arrangement where the frame surrounding the cap is also made of firebrick does not result in any undue wear of either the frame opening or the cap-seating, since they are both made of similar materials. On the other hand, if the cap were made exclusively of cast iron or steel as in prior art arrangements, invariably the frame opening is made of metal, too, to prevent undue wear; such arrangement results in excessive heat losses because of thermal conduction both through the metallic cap and the surrounding metallic frame. The present invention makes it feasible to construct a ceramic frame opening and a cap substantially made of ceramic material, at the same time retaining the advantages and desirability of magnetic hoisting of the cap during loading and unloading of the coke oven.